1. Field of Invention
The present invention relates to an analog circuit, and particularly to a bandgap reference circuit.
2. Description of the Related Art
Voltage reference circuits and current reference circuits are widely used in analog circuits. The refence circuits provide a DC level with a negligible correlation to process parameters. For example, a bias current of a differential pair circuit must rely on a reference circuit to be generated. In the differential pair circuit, the generated bias current in reverse affects the voltage gain and noise of the circuit. Similarly, in an analog-to-digital converter (ADC) and a digital-to-analog converter (DAC), the entire input/output ranges must be defined by a reference circuit.
Normally, to obtain a stable reference voltage level unvaried with temperature, a PTC (positive temperature coefficient) voltage must be used to compensate a NTC (negative temperature coefficient) voltage, as shown in FIG. 1A, a schematic principle drawing of a conventional bandgap reference circuit. In FIG. 1A, the voltage between base and emitter VBE of the bipolar transistor Q is a NTC voltage. In the circuit, a voltage proportional to absolute temperature (Kelvin degree) is multiplied by K for compensating the voltage VBE with a NTC (negative temperature coefficient). FIG. 1B is a schematic layout of the conventional bandgap reference circuit in FIG. 1A. The circuit in FIG. 1B includes bipolar transistors Q101 and Q102, resistors R101, R102 and R103, and an operational amplifier A100.
Restricted by semiconductor processes, the conventional bandgap reference circuit in FIG. 1B is not capable of providing a lower-voltage reference level output (for example, a level less than IV). To overcome the problem, another conventional lower-voltage bandgap reference circuit was provided, as shown in FIG. 2. The lower-voltage bandgap reference circuit in FIG. 2 includes bipolar transistors Q201 and Q202, P-FETs (P-type field effect transistor) M201, M202 and M203, resistors R201, R202, R203 and R204, and an operational amplifier A200. The circuit uses the scheme of FIG. 1B to produce a stable voltage VR1, which is coupled to the gates of the P-FETs M201, M202 and M203 for forming a current mirror. In the end, the output current from M203 flows into the resistor R204 for producing a reference voltage level VREF.
Yet, there has not been an integrated bandgap reference circuit to produce both a higher-voltage and a lower-voltage so far. To meet such requirement in some applications, a higher-voltage bandgap reference circuit and a lower-voltage bandgap reference circuit are disposed simultaneously, which leads an oversized circuit size.